The present invention generally relates to a document scanning apparatus, and more particularly to a document scanning apparatus having an image signal correcting function which may be applied to an image processing part of a facsimile or the like.
Conventionally, in a document scanning apparatus of a facsimile or the like, an optical unit has a scanning position where light is scanned over a document at a halfway portion of a document transport path between a pair of feeding rollers and a pair of ejection rollers for reading image data from the document. At this scanning position, light emitted from a light source in the optical unit is scanned over a surface of the document which is transported along the sheet transport path. One line of the image information in the document is scanned at a time by the optical unit in a main scanning direction, the document is transported a fixed distance along the sheet transport path by rotation of the feeding rollers and/or the ejection rollers.
FIGS. 1 and 2 show an essential part of a prior document scanning apparatus. In the essential part of the document scanning apparatus shown in FIGS. 1 and 2, a document 1 is transported along a transport path 2 between a pair of feeding rollers 3 and a pair of ejection rollers 4. An optical scanning unit (not shown) has a scanning position 5 at a halfway position of the transport path 2 where a light beam emitted from a light source is scanned over a surface of the document 1 in a main scanning direction to read image information from the document 1 per each scan line.
For example, when a document 1 with an upwardly curled front edge portion is transported along the transport path 2, as shown in FIG. 1, the front edge of the document 1 comes unsuitably in contact with an upper roller of the ejection rollers 4 at a position indicated by "A" in FIG. 1. And, the document 1 is fed by the feeding rollers 3 as much as a given distance "a1" along the transport path, each time a scanning is made by the optical unit over the document 1 at the scanning position 5 in one scanning line in a main scanning direction. And, after one main scanning is made by the optical unit, the ejection rollers 4 are rotated as much as a given amount of rotation equivalent to the given transport distance al along the transport path. And, the front edge of the document 1 is moved to a position of the upper roller 4 as indicated by "B" in FIG. 1 and it is transported a distance "a2" along the transport path, which is slightly smaller than the proper transport distance "a1". Accordingly, the document 1 during transport has slack portions between the feeding rollers 3 and the ejection rollers 4, and the quantity x1 of transport of the document 1 at the scanning position 5 may be represented by a formula: a2.ltoreq.x1.ltoreq.a1. In this case, there is a problem in that the document 1 is deficient in the transport distance x1 at the scanning position 5, and a scanning of image information from the document 1 is not carried out with a proper transport distance of the document 1.
In FIG. 2, in order not to allow slack in the document 1 at the scanning position 5 in normal operation, a transport distance of the document 1 given by the ejection rollers 4 is slightly greater than a transport distance given by the feeding rollers 3 so that the document 1 in the transport path is kept under tension. And, the document 1 should be kept under tension along the transport path between the feeding rollers 3 and the ejection rollers 4 until the rear edge of the document 1 separates from the feeding rollers 3. However, once the rear edge of the document 1 separates from the feeding rollers 3, the document 1 returns back to its original condition, or a slack condition as illustrated in FIG. 2, due to the curling tendency of the document 1. As a result, the rear edge of the document 1 in this case is fed by the ejection rollers 4 as much as a transport distance "a3" along the transport path as shown in FIG. 2, which is slightly greater than the proper transport distance "a1", and the quantity "x2" of transport of the document 1 at the scanning position 5 may be represented by a formula: a1.ltoreq.x2.ltoreq.a3. In this case, there is a problem in that the transport distance of the document 1 at the scanning position 5 is excessive and a scanning of image information from the document 1 is not carried out with a proper transport distance.
FIG. 3A shows an example of pixels representing an inclined line reproduced from an original image, the pixels being normally outputted by the prior document scanning apparatus when a document is fed by a proper transport distance along the transport path. However, in the above described case in which an excessive transport distance along the transport path takes place as shown in FIG. 2, part of the reproduced image often lacks pixels when compared with the normally outputted pixels as shown in FIG. 3A, this lacking part being indicated by a letter "Y" in FIG. 3B, where a few pixels disappear due to the excessive transport distance. For this reason, the quality of reproduced images supplied by the prior document scanning apparatus deteriorates.